I came across an issue that was well worth documenting and sharing. 

Some of the symptoms that you may see for this issue include:

OWA returns the following error

 

HTTP 1.1 503 Service Unavailable

 Also, you can get the following.

Active Server Pages error 'ASP 0201'

 Invalid Default Script Language

 The default script language specified for this application is invalid.

 

The resolution was to set the permissions on the HKEY_CLASSES_ROOT key back to the default.  They had been modified and set to just Everyone. 

The default permissions for the key are:

 

Administrators - Full Control, Read

Authenticated Users - Read

CREATOR OWNER – Special Permission (Inherited)

Server Operators – Read (Explicit), Special Permissions (Inherited)

SYSTEM – Full Control, Read

 

During initialization, the Exchange OLE DB provider (ExOLEDB.dll) scans the HKEY_CLASSES_ROOT hive to identify registered file types. If any subkey has a default value that exceeds 259 characters, or if there is a discretionary access control list that is not valid on one of the keys, ExOLEDB may quit unexpectedly and prevent the OLE DB provider from initializing.

To be complete, there are other causes of this error.  Probably the most common one is detailed in KB 296626.